Polyester is the name usually given to those synthetic polymers which are the condensation products of dicarboxylic acids and dihydroxy alcohols, particularly either dimethyl terephthalate or terephthalic acid condensed with ethylene glycol. Polyester has obtained widespread use in magnetic tapes, bottles, nonwoven textiles, films, and fibers.
In commercial polyester manufacturing processes, for example those for manufacture of polyester chip, fiber, film, or packaging, one of the last steps in the process is to filter polyester in the molten or liquid state through a series of fine filters to remove the various impurities or other solid particulates which would detract from the appearance and performance of extruded polyester.
In one type of mechanical filtration system, the liquid polyester is directed through long tubular filters arranged in bundles. Each of the filters in turn is formed of a number of fine metal fibers arranged in random fashion, somewhat analogous to a nonwoven textile. These fibers are in turn held in place in the tubular filter elements by a fine mesh screen. The filtration and cleaning conditions under which these elements operate tend to be rather harsh. In particular, such filter elements are exposed to temperatures on the order of about 300.degree. C. and pressures of up to 1000 pounds per square inch (psi) for prolonged periods of time. They are also exposed to the polyester itself, and to related materials including unreacted precursors such as glycols, esters and acids.
As might be expected, such filter elements can eventually become clogged and need to be either cleaned or replaced, with cleaning typically preferred because of the relatively high cost of each element. One preferred cleaning process comprises boiling the elements in an appropriate solvent such as triethylene glycol until satisfactory conditions are reestablished.
Other commonly produced polymers are also filtered with the same type of filter elements. These include thermoplastic polymers such as polyethylene, polypropylene and polystyrene. The chemical nature of these polymers is such that the preferred method of cleaning the filter elements is a high temperature "burn-out" process in which the residual polymer is thermally degraded and then the element is washed appropriately. For example, a typical process would involve heating the elements to a temperature of between about 300.degree.-450.degree. C. for a period of four or five hours. The specific temperature would depend upon the particular polymer being cleaned from the element. Polyethylene and polypropylene will degrade at the lower temperatures in this range, while polystyrene will only degrade when the higher temperatures are reached.
Because of the relatively harsh filtration and cleaning conditions to which the filter elements are exposed, they often develop flaws, particularly when the fine mesh screen breaks at a particular point thereby producing a hole. One such small hole can eliminate an entire filter element from further use. As mentioned earlier, the filter elements are relatively expensive making repair desirable. A number of repair techniques have accordingly been attempted to date, but generally without success. One technique includes the use of fused powdered metals to repair the filter elements, but the method raises two problems. First, the patches tend to crack under use and cleaning. Second, the high temperatures required to fuse the metal can adversely affect the filter media, which are often formed from stainless steel.
Accordingly, to date there exists no satisfactory technique for repairing these filter elements and consequently they must generally be replaced at significant costs, even where the flaws in them are relatively small and straightforwardly detectable.